Eddy current printer apparatus

ABSTRACT

Printer apparatus is disclosed in which an image carrier is forced against the material to be printed by a dynamic impact rather than a static pressure, said impact being distributed uniformly over a thin metallic plate repelled from a pulsed current source by induced eddy currents. The apparatus eliminates the need for massive structures and permits printing on irregularly shaped surfaces, or on surfaces too large to be handled in a static press.

' United States Patent KO m Dec. 10, 1974 [54] EDDY CURRENT PRINTER APPARATUS 3,448,687 6/ 1969 Pittenger et a1. l0l/426 3,486,449 12/1969 Levine [75] lnvemol- Henry Kmm, Wayland Mass- 3,526,704 9/1970 Heller 101/426 x 73 Assignee; Massachusetts Institute of 3,526,708 9/1970 Leatherman 101/426 X Technology, Cambridge Mass. 3,587,455 6/1971 Ch1ldress et al. 101/111 X [22] Filed: 1974 Primary Examiner-Robert E. Bagwill [21] Appl. No.: 435,929 Assistant ExaminerEdward M. Coven Attorney, Agent, or FirmArthur A. Smith, Jr Related U.S. Application Data Robert Shaw; M artin M. Santa [63] Continuation of Ser. No, 189,618, Oct. 15, 1971,

abandoned, which is a continuation-in-part of Sen No. 849,126, Aug, 11, 1969, abandoned, which is a [57] ABSTRACT continuation-in-part of Ser. No. 761,048, Sept. 20, 1968, Pat 3,567,026. Printer apparatus is disclosed 1n which an image carrier is forced against the material to be printed by a 52 us. (:1 101/316, 101 /426, 101/111 dynamic impact rather than a Static Pressure Said 51 Int. Cl B411 1/06 P being distributed uniformly Over a thin metallic [58] Field of Search 101/316, 426, 111, 93 0; Plate repelled from a Pulsed current Source by 197/1R;340/173.346/77, 135; 252/316 duced eddy currents. The apparatus eliminates the need for massive structures and permits printing on [56] References Cited irregularly shaped surfaces, or on surfaces too large to UNITED STATES PATENTS be handled in a static press.

3,182,591 5/1965 Carlson .L 101/426 17 Claims, 5 Drawing Figures A-C INPUT RECTIFIER \GROUNC Pmmm w 3.853.055

SK 1 0f 3 55 ffi I 54 5s )1 CAPACITOR BANK CHARGING RESISTOR RECTIFIER A-C INPUT PATEMEUSECIOISH V 3,853.05?)

' sum 20! 3 I l n.

EDDY CURRENT PRINTER APPARATUS The invention herein described was made in the course of work performed under a contract with the ILS. Air Force Office of c i e r rtific Research.

This is a continuation of application Ser. No. 189,618 filed Oct. 15, 1971, now abandoned, which was a continuation-in-part of application Ser. No. 849,126 filed Aug. 1 l, 1969, now abandoned which is, in turn, a continuation-in-part of application Ser. No. 761,048, filed on Sept. 20, 1968 (now US. Pat. No. 3,567,026) and which was filed as a result of a requirement for restriction.

The present invention relates to printers and, more particularly, to printers employing eddy current means to provide the force needed to effect printing.

Traditional printing is accomplished by applying a static pressure between an image carrier and a material to be printed. The image carrier may be an inked plate or drum having raised characters (letterpress printing) or chemically differential characters (offset printing), or it may be electrostatically charged so that dry pigment particles, called a toner, adhere to form the image (xerography). The printing pressure is applied between massive structures mechanically forced together, one to support the image carrier, the other called a platen, to support the material to-be-printed. Both the plate and the platen may be planar (flat bed printing), or one or both may be cylindrical (rotary printing).

A principal object of the-present invention is to make use of an image carrier that is conventional in type, that is, an inked letterpress or offset plate or a xerographic image carrier toned with dry powdered pigment, but to replace the static pressure used in conventional presses by a dynamic impact distributed uniformly over the entire printing surface, the impact being adequate to effect printing but of sufficiently short duration to eliminate the need for a massive structure to hold the plate and the platen with respect to each other or to ensure their rigidity.

In the apparatus hereinafter described in detail a uniformly distributed impact force is delivered to a thin metal plate, hereinafter called an impact plate, by a relatively thick, high inertia plate, called a driver platewhich consists ofa non-conducting material containing an imbedded pattern of electrically conducting wire, the impact plate resting on the driver plate and being repelled from the latter when a sharp pulse of electric current is passed through the imbedded wires. The current pulse is most expediently obtained from a charged capacitor bank, and the repulsive impact is produced by eddy currents induced in the impact plate by the magnetic field of the current flowing in the driver plate. It is the reaction between these eddy currents and the magnetic field which produces the repulsive force. The repulsion may equivalently be thought of as occurring between the currents flowing in the driver plate and the oppositely directed image currents they induce in the impact plate. The impact delivered to the impact plate in several embodiments, is absorbed by a third or backing plate, which is substantially more massive than the impact plate, but not so massive as the platen used in conventional static pressure printing. Printing is accomplished between the impact plate substantially more massive than the impact plate, but not so massive as the platen used in conventional static pressure printing. Printing is accomplished between the impact plate and the backing plate, the exact geometry and arrangement of the various components depending upon the use in question. Impact printing represents the first significant mechanical innovation since Gutenberg. It can be compared to driving a nail into a two-by-four by the impact of a hammer, as contrasted to the static method of forcing it in by means of a screwjack or a hydraulic press. It is important to note and it is a still further object to provide an impact printer in which the separate components thereof are simply held in juxtaposition by any suitable means, the impact being absorbed by inertia of the driver plate and the backing plate without the need for a massive structure to absorb the printing pressure and to maintain rigidity.

Impact printing according to the invention eliminates several shortcomings and limitations inherent in static pressure printing. One of those involves the flat-bed letterpress, still considered the only means to achieve high quality impressions. Since the use of a resilient platen, or a rubber mat, causes a deterioration of print quality, it is necessary to use very massive structures to prevent distortion of the plate and platen and ensure a uniform distribution of the printing pressure in such presses. Even in a modern flat bed press, it is necessary to use a multitude of shims, adjusted by a tedious process of trial and error, in order to achieve a uniform impression. Moreover, since the mass of a press increases about as the cube of its linear dimension, flat bed printing is restricted to relatively small sheet sizes. This circumstance, together with the slow speed inherent in massive structures, has made flat bed printing prohibitively expensive by modern standards. The impact printer eliminates the need to distribute the printing force from two points over the entire surface by gener ating a uniformly distributed impact force in a thin metal plate which can adapt to the shape of the backing plate, without the need for shims to achieve a planar surface. A further object, therefore, is to provide a printer capable of furnishing high quality print in simple, light-weight construction.

Another limitation of static pressure printing is its inability to print on objects which are irregularly shaped, or too delicate to withstand the printing force. An example is the printing of molded containers such as plastic bottles, that is, containers which cannot be fabri cated from pre-printed flat stock as are beer cans, for example. The printing of fabricated containers has been a problem for some time, and has achieved increased importance recently in connection with recycling of used containers. Accordingly, a still further object is to provide a printer which can print, impress, or otherwise apply indicia to a curvilinear or other nonplanar or irregular surface, one in which, in some circumstances, the object to-be-printed serves also the platen function, thereby eliminating the platen from the apparatus.

Other objects are discussed in the description to follow and are.particularly delineated in the appended claims.

By way of summary, the objects of the invention are attained in an eddy-current impact printer that includes a thin, flexible conductive impact plate or sheet to furnish the impact force required to effecting imprinting or otherwise marking of an object. A driver plate is positioned adjacent one major surface of the impact plate. Electric coil means is disposed within the driver plate near the surface thereof adjacent the impact plate and has closely-spaced electrical wires embedded in substantially the whole area of that surface of the driver plate, the contour of the imbedded wires, the contour of said driver plate surface and the contour of the impact plate ideally being similar so that all portions of each are equidistant from all adjacent portions of the other. A source of electric pulse energy is connected to energize the coil means to create a pulsed magnetic field, the field being oriented to enter the impact plate at said major surface, thereby to induce eddy currents therein. Interaction between the magnetic field and the eddy currents creates a force upon the impact plate which is propelled toward the object, and indicia on the impact plate, for example, are thereby applied to the object. A backing plate or platen may be used in those instances in which the object to-be-printed is not sufficiently massive to absorb the momentum of the impact plate. In any event, the driver plate must be sufficiently massive to effect acceleration of the impact plate without transferring any appreciable force to the support structure of the printer and the backing plate, if used, must be sufficiently massive to stop the impact plate without transferring any appreciable force to the support structure.

The invention will now be described with reference to the accompanying drawing, in which:

FIG. 1 shows, schematically, an isometric view of a flat-bed printer embodying the present invention, the view being exploded to show more clearly the relative positions of the various parts;

FIG. 2 is an isometric view, also in exploded form, of a modification of the apparatus of FIG. 1;

FIG. 3 is an isometric view ofa further modification, particularly to show a type of mechanism useful for imprinting, embossing, or otherwise applying some sort of marking to an uneven surface;

FIG. 4 is an end view, diagramatic in form, of a further embodiment; and

FIG. 5 is a view, on an enlarged scale, taken on the line 5-5 in FIG. 2 looking in the direction of the arrows.

Referring now to FIG. 1, the eddy current printer apparatus there shown comprises a thin flexible conductive sheet or impact plate 54 of aluminum or other highly conducting material. In the non-exploded assembled apparatus the sheet rests upon the flat upper portion of a driver plate 56 which, as later explained, applies a sharp force to the sheet to propel it upward. In the actual apparatus the sheet 54 is disposed adjacent to and close-spaced from a piece of paper 55 or other surface to-be-printed and in FIG. 1 has the same fiat contour as that surface. Between the sheet 54 and the surface to-be-printed 55, there is a printing plate 52 which contains the indicia 53 to be transferred to the paper. Electric eddy current means, which is operable to introduce a magnetic eddy current field to enter the conductive sheet 54 at the lower major surface thereof, provides the printing force needed to imprint. The field must have a skin depth that is comparable to or less than the thickness of the sheet 54 to induce eddy currents therein. In order to provide the necessary force to the sheet 54 for the present purposes, it has been found that the skin'depth must be no greater than about three times the thickness of the impact plate and usually about the thickness of the plate; the term comparable is used herein to denote such a skin depth. The

electric eddy current means comprises the driver plate 56, which is made of a non-conducting material containing an imbedded electric conductor 57 which meanders substantially over the whole region of the driver plate near the driver plate surface adjacent the conductive sheet 54 so that eddy currents are produced uniformly substantially throughout the conductive sheet. The driver plate 56 is with relation to the conductive sheet relatively massive in order that the recoil momentum energy is absorbed mostly by the driver plate in the imprinting process. The force applied thereby to the sheet 54 by the driver plate 56 is an impulse, similar to 'a hammer blow and the effect is that the sheet 54 is propelled toward the surface to-be-irnprinted with ballistic-like speed. Thus, the interactions are high energy, short time, percussive in nature,.thereby allowing reaction forces to be absorbed, first, by a relatively massive driver plate and, second, by a relatively massive backing plate 53 against which the paper 55 is forced in the printing process. The backing plate 53, it will be understood, serves the function of a platen in more conventional printing apparatus. It is to be observed that in FIG. 1 only the active printing members are shown and that the supporting structures necessary are not included. It should be quite apparent, however, from the present discussion, that the supporting framework is of a secondary nature and as explained, need not be a heavy structure.

To operate the apparatus, a capacitor bank previously charged to a suitable voltage (which depends on the size of the apparatus, and the resulting inductance of the winding), is discharged through the conducting wire 57 imbedded in the driver plate by closing a discharge switch 58, preferably an electronically operated switch such as an ignition tube or a triggered spark gap. The resulting current pulse must be of sufficiently-short duration that the skin depth of the magnetic field produced, as mentioned, is comparable to the thickness of the aluminum sheet 54, that is to say, only a small fraction of the magnetic field penetrates through the aluminum sheet. Most of the magnetic field is, thus, effective in inducing eddy currents in the aluminum'sheet, and these currents are distributed and timed in such a way as to oppose the change in magnetic field which attempts to penetrate the sheet. These eddy currents react against the pulsed magnetic field which generates them and thereby force the impact plate or sheet 54 in the upward direction, the force being proportional to the rate of change of the pulsed magnetic field.

Satisfactory results will be obtained, for example, if the pulse has a rise-time of to 300 microseconds, and the impact plate 54 has thickness of 2 millimeters (0.080 inches). These parameters can be varied as expedient, provided only that the impact plate is approximately as thick as one skin depth at the pulse risetime selected as mentioned above, but it can be slightly (i.e., two or three times) larger which decreases instantaneous forces but can result in increased energy delivered because of the time span over which the force is delivered. The pulse in question is of such short duration that the backing plate 53 is adequately held by its own inertia, and no conventional type of press yoke or other structural members are required to secure it. It is one of the advantages of this apparatus that the printer can be made as large as desired without any disproportionate increase in its bulk or strength, as is the case in conventional mechanical printing presses.

The coil 57 can be replaced by two coils which are discussed in said US. Pat. No. 3,567,026, to furnish a double pulse of the type therein discussed, if necessary for high eddy current forces. In such event the coils can each be connected to two capacitor banks of the type shown herein with appropriate switching and interconnection to provide current pulsing at the proper phase.

FIG. 2 illustrates a flat bed press which is particularly adapted to impact printing. In FIG. 2 the electrically conductive sheet 54 is shown formed into a continuous loop or flexible belt l'comprising, as best shown in FIG. 5 which is a partial end view of a composite belt 1, several made up of bands, 7, 8 and 9, the uppermost band 7 constituting the image carrier. That uppermost band 7 may be a thin letterpress plate or offset plate, a xerographic image carrier or a carrier of other indicia to-betransferred to a paper or other surface to-beimprinted. Thus, the image carrier need not, and in some situations cannot, be an electrical conductor. Below this image carrier band 7 are the bands 8 and 9 which have high electrical conductivity and which, together, constitute the impact plate and printing plate, in combination. The conductive bands must have sufficient combined thickness to satisfy the electrical skin depth requirements and must be flexible enough to adapt to any curvature required. The composite printing belt I is guided over the driver plate labeled 2 by the two rollers 3 and 3' in such a way that it traverses the upper surface of said driver plate and returns along its lower surface. Inking of the image carrier (or pigmentation in the case of xerography) occurs as the carrier passes across the lower surface by action of two inking rollers 4 and 5, the lower of which is immersed in an ink trough 6. The paper 10 is carried past the image carrier between rolls 11 and 12, passing along the lower surface ofa backing plate 13, again relatively massive. The frame portions of the eddy-current printer illustrated in FIG. 2 are all labeled 14. Again, the structural parts are needed only to preserve the relative positions of the active printer portions of the apparatus and not to absorb impact or other imprinting forces which are absorbed by either the printing plate 2 or the backing plate 13.

The paper 10 is passed, in the direction of the arrow designated A, over the image carrier belt 1 in the form of a continuous ribbon, at a speed synchronized with the speed of the belt. It makes only loose contact with the image carrier. Whenever the paper and image carrier have advanced by the length of one imprint, the driver plate 2 applies a printing impact to the composite belt 1, forcing it and the paper against the backing plate 13. The device can be operated continuously without stopping at the speed of a rotary press, and yet achieves printing quality of a flat bed press. An additional advantage is the fact that the printing plate can be manufactured in the form of a thin sheet, making it signifcantly less expensive than the massive curved cylindrical plates used in conventional rotary printing.

The apparatus of FIG. 3 illustrates an impact printer adapted to print irregularly shaped surfaces such as the molded plastic bottle shown at 15. The composite printing belt 1 in this case passes over an arrangement of three rollers 16, 17 and 19 which support it elastically enough to permit an appreciable deflection of the belt between rollers. The image carrier in this case forms the inner surface of the composite belt 1, and it is inked continuously by the upper support roller 17 in the manner shown. The driver plate 2 is located beneath the horizontal portion of the belt; when a pulse is applied, it forces the belt 1 upward against the irregularly shaped object to-be-printed, here the plastic bottle l5. Objects to be printed are moved past the printing surface continuously by means not shown, in the direction parallel to the axes of the guiding rollers. They are held a slight distance above the printing surface, so that the belt must deflect slightly upward to make contact. The contact is sufficiently brief to allow continuous motion of the object being printed without causing smudging. In most practical cases the bottle and its content are sufficiently massive toabsorb the printing impact without undue recoil or damage and without the need of a backing plate. The driving mechanism contains a roller-type chain 20 which drives a roller 18, held by a spring 21 against the roller 17. Again structural parts are numbered 14 and the further elements shown in FIG. 3 have applied to them the numbers used in FIG. 2.

The embodiment of FIG. 4 is quite similar to that of FIG. 3 and some elements are given identical numbers. The thin walled elastic cylindrical or closed-loop belt 1 is deformed, driven and guided by rollers 22, 23 and 24, the roller 24 being driven by means not shown to revolve the image carrying belt 1 past the driver plate 2 to imprint the plastic container 15. The image carrying belt 1, in FIG. 4, is curved to the same contour as the container 15 in the region of the container by interaction between the elastically deformable cylinder 1 and the rollers 22, 23 and 24. Rotary motion is in the direction of the arrow A, as before. An eddy-current field is provided by conductors 28 which are imbedded in the relatively massive driver plate 2. Inking is accomplished by use of rollers 25, 26 and 27 in conjuction with the ink container 6.. The belt 1 can be formed from a thin conductive sheet which has on it the imprint indicia and which has been formed into a cylinder and spot welded. The elastic character of the belt 1 allows it to be propelled by the eddy-current force toward the immediately adjacent surface of the container 15, with ballistic-like speed, thereby to imprint, impress, emboss or otherwise mark the container surface.

The arrangements shown herein are intended only to clarify the principles involved. Modifications in geometry can readily be made to accommodate various requirements. Thus it is possible, for example, to use a belt configuration which permits printing both sides of a bottle or other container simultaneously by the use of two synchronized driver plates. Analogous modification can also be made in the flat bed printer embodiment. It may be economically expedient, for example, to print both sides of a paper ribbon simultaneously by using two synchronized driver plates operating two belts, like the belt 1, one at each surface of the paper ribbon.

The letters or other indicia whose image is impressed or otherwise transferred to a paper or other surface need not be electrically conductive, and, indeed, in the case of xerography processes cannot be, since the impelling impetus is supplied by either a separate conductive impact plate or a separately conductive portion thereof. Thus, the indicia receive a mechanical impelling force and, since each letter or other indicium occupies only a very small portion of the printing plate or image carrier and therefore all move together as a unit 7 therewith, rigidity of individual letters is not a prerequisite. Again, this arrangement allows the use of xerography and offset printing; broadly the invention herein disclosed allows the use of raisedor lowered letan impact absorbing surface against which printing can be effected, a thin homogeneous conductive sheet disposed adjacent to and closely spaced from the impact absorbing surface and having a contour similar'to the contour of the impact absorbing surface, electric eddycurrent means operable to cause a magnetic eddycurrent field to enter the conductive sheet at one major surface thereof, thereby inducing an eddy current in the conductive sheet, which eddy current interacts with the field to provide a force to propel the conductive sheet toward the backing plate, a printing plate positioned between the conductive sheet and the backing plate, said printing plate being provided with indicia and being operable to move toward the backing plate under the impetus of the conductive sheet thereby to transfer indicia on said printing plate to a paper or the like when located between the printing plate and the impact surface of the backingplate, the electric eddycurrent means comprising a driver plate of nonconductive material containing an embedded conductor which meanders over the whole of one surface region thereof so that eddy currents are induced substantially uniformly throughout the conductive sheet, both the driver plate and the backing plate being relatively massive compared to the mass of the conductive sheet so that the recoil energy of the conductive sheet is absorbed mostly by the driver plate and the deceleration energy upon impact thereof is absorbed mostly by the backing plate, the driver plate, the conductive sheet and the backing plate being closely spaced from one another so that the eddy currents provide an impulse distributed substantially uniformly over the conductive sheet thereby forcing the conductive sheet toward the backing plate.

2. A printer as claimed in claim 1 in which both the backing plate and the sheet are substantially planar and in which the conductive sheet and the pulsed driver plate are oriented parallel to one another.

3. A printer as claimed in claim 2 in which the electriceddy-current means includes a source of fast-rising pulsed electric current connected to the embedded conductor to provide a fast-rising magnetic field.

4. A printer as claimed in claim 3 in which the risetime of the pulse is the order of 100 microseconds and in which the sheet thickness is the order of 2 millimeters. v

5. In an eddy-current printer, a thin flexible, electrically conductive homogeneous sheet, electric eddycurrent means adjacent to said conductive sheet and operable to cause a magnetic eddy-current field to enter the conductive sheet at one major surface thereof thereby inducing an eddy current in the conductive sheet, which eddy current interacts with the field toprovide a force to propel the conductive sheet toward stantially over the whole of one surface region thereof so that eddy currents are induced substantially uniformly throughout the conductive sheet, the driver plate being relatively massive compared to the mass of the conductive sheet so that the recoil energy of the conductive sheet is absorbed mostly by the driver plate, the driver plate and the conductive sheet being closely spaced from one another so that the eddy currents provide an impulse distributed substantially uniformly throughout the entire portion of the conductive sheet adjacent the plate, thereby propelling the conductive sheet and the printing plate toward said surface with sufficient force to effect printing on said surface.

6. In an eddy-current printer, a thin, flexible, relatively large area, electrically conductive homogeneous sheet, electric eddy-current means adjacent to said conductive sheet and operable to cause a magnetic eddy-current field to enter the conductive sheet to induce eddy currents therein, which eddy currents interact with the field to provide a force to propel the conductive sheet toward a surface to be imprinted, a printing plate positioned on the opposite side of the conductive sheet from the eddy-current means, said printing plate being provided with indicia and being operable to move toward and to strike said surface under the impetus of the conductive sheet, the electric eddy-current means comprising a driver plate of non-conducting material containing an embedded electrical conductor disposed substantially through the whole of the plate so that eddy currents are induced substantially uniformly throughout the conductive sheet, the driver plate being relatively massive compared to the masss of the conductive sheet so that the recoil energy of the conductive sheet is absorbed mostly by the driver plate, the driver plate and the conductive sheet being closely spaced from one another so that the eddy currents provide an impulse distributed substantially uniformly throughout the entire portion of the conductive sheet adjacent the plate thereby propelling the sheet and the printing plate toward said surface with sufficient force to effect printing on said surface.

7. An eddy-current printer as claimed in claim 6 in which the electrically conductive sheet is formed into a continuous loop and the outer surface of which contains indicia to be transferred to the surface to-beimprinted, the sheet being driven along a closed path alternately to bring the indicia into the region of the surface to be imprinted and to a region where the indicia can be inked, and which contain means for inking.

and operable to cause a magnetic eddy-current field to enter the conductive belt to induce an eddy current in said portion, which eddy current interacts with the field to provide a force to propel the conductive belt toward a surface to-be-imprinted, printing means positioned to move toward and to strike the surface to-be-imprinted under the impetus of the conductive belt, the electric eddy current means comprising a driver plate of nonconducting material containing an embedded electrical spaced from one another so that the eddy currents providean impulse distributed substantially uniformly over the entire said portion of the conductive belt thereby forcing that portion of the conductive belt toward the surface to-be-imprinted.

11. An eddy-current printer as claimed in claim in which the conductive belt and printing means are associated in a composite structure made of a plurality of thin closed-loop bands, the band adjacent the surface to-be'imprinted being the image carrier of the printer and the other of the bands having high electrical con ductivity.

12. An eddy-current printer as claimed in claim 11 that includes means for applying a transferable image to the image carrier.

13. An eddy-current printer as claimed in claim 12 in which the image carrier contains indicia and in which the means for applying a transferable image comprises inking means.

14. An eddy-current printer as claimed in claim 13 that includes a backing plate, the surface to be imprinted being positioned between the driver plate and the backing plate to be pressed therebetween upon the introduction of electric current to the electrical conductor.

15. An eddy-current printer as claimed in claim 14 that includes a source of pulse current electrically connected to said conducor.

16. An eddy-current printer as claimed in claim 15 which includes roller means adapted to drive the belt along a closed-loop path, inking occurring at one position of the belt along the path and printing occurring at another position.

17. An eddy-current printer as claimed in claim 10 said conductive belt being flexible enough to conform to varying curvature on the surface to-be-imprinted under the impetus of said impulse. 

1. In an eddy-current printer, a backing plate having an impact absorbing surface against which printing can be effected, a thin homogeneous conductive sheet disposed adjacent to and closely spaced from the impact absorbing surface and having a contour similar to the contour of the impact absorbing surface, electric eddy-current means operable to cause a magnetic eddy-current field to enter the conductive sheet at one major surface thereof, thereby inducing an eddy current in the conductive sheet, which eddy current interacts with the field to provide a force to propel the conductive sheet toward the backing plate, a printing plate positioned between the conductive sheet and the backing plate, said printing plate being provided with indicia and being operable to move toward the backing plate under the impetus of the conductive sheet thereby to transfer indicia on said printing plate to a paper or the lIke when located between the printing plate and the impact surface of the backing plate, the electric eddy-current means comprising a driver plate of non-conductive material containing an embedded conductor which meanders over the whole of one surface region thereof so that eddy currents are induced substantially uniformly throughout the conductive sheet, both the driver plate and the backing plate being relatively massive compared to the mass of the conductive sheet so that the recoil energy of the conductive sheet is absorbed mostly by the driver plate and the deceleration energy upon impact thereof is absorbed mostly by the backing plate, the driver plate, the conductive sheet and the backing plate being closely spaced from one another so that the eddy currents provide an impulse distributed substantially uniformly over the conductive sheet thereby forcing the conductive sheet toward the backing plate.
 2. A printer as claimed in claim 1 in which both the backing plate and the sheet are substantially planar and in which the conductive sheet and the pulsed driver plate are oriented parallel to one another.
 3. A printer as claimed in claim 2 in which the electric eddy-current means includes a source of fast-rising pulsed electric current connected to the embedded conductor to provide a fast-rising magnetic field.
 4. A printer as claimed in claim 3 in which the rise-time of the pulse is the order of 100 microseconds and in which the sheet thickness is the order of 2 millimeters.
 5. In an eddy-current printer, a thin flexible, electrically conductive homogeneous sheet, electric eddy-current means adjacent to said conductive sheet and operable to cause a magnetic eddy-current field to enter the conductive sheet at one major surface thereof thereby inducing an eddy current in the conductive sheet, which eddy current interacts with the field to provide a force to propel the conductive sheet toward a surface to be imprinted, a printing plate positioned on the opposite side of the conductive sheet from the eddy-current means, said printing plate being provided with indicia and being operable to move toward and to strike said surface under the impetus of the conductive sheet, the electric eddy-current means comprising a driver plate of non-conducting material containing an embedded electrical conductor which meanders substantially over the whole of one surface region thereof so that eddy currents are induced substantially uniformly throughout the conductive sheet, the driver plate being relatively massive compared to the mass of the conductive sheet so that the recoil energy of the conductive sheet is absorbed mostly by the driver plate, the driver plate and the conductive sheet being closely spaced from one another so that the eddy currents provide an impulse distributed substantially uniformly throughout the entire portion of the conductive sheet adjacent the plate, thereby propelling the conductive sheet and the printing plate toward said surface with sufficient force to effect printing on said surface.
 6. In an eddy-current printer, a thin, flexible, relatively large area, electrically conductive homogeneous sheet, electric eddy-current means adjacent to said conductive sheet and operable to cause a magnetic eddy-current field to enter the conductive sheet to induce eddy currents therein, which eddy currents interact with the field to provide a force to propel the conductive sheet toward a surface to be imprinted, a printing plate positioned on the opposite side of the conductive sheet from the eddy-current means, said printing plate being provided with indicia and being operable to move toward and to strike said surface under the impetus of the conductive sheet, the electric eddy-current means comprising a driver plate of non-conducting material containing an embedded electrical conductor disposed substantially through the whole of the plate so that eddy currents are induced substantially uniformly throughout the conductive sheet, the driver plAte being relatively massive compared to the masss of the conductive sheet so that the recoil energy of the conductive sheet is absorbed mostly by the driver plate, the driver plate and the conductive sheet being closely spaced from one another so that the eddy currents provide an impulse distributed substantially uniformly throughout the entire portion of the conductive sheet adjacent the plate thereby propelling the sheet and the printing plate toward said surface with sufficient force to effect printing on said surface.
 7. An eddy-current printer as claimed in claim 6 in which the electrically conductive sheet is formed into a continuous loop and the outer surface of which contains indicia to be transferred to the surface to-be-imprinted, the sheet being driven along a closed path alternately to bring the indicia into the region of the surface to be imprinted and to a region where the indicia can be inked, and which contain means for inking.
 8. An eddy-current printer as claimed in claim 6 in which the conductive sheet is a loop and is adapted to travel along a closed path.
 9. An eddy-current printer as claimed in claim 8 having means for applying to the sheet indica to be transferred to said surface.
 10. In an eddy-current printer, a thin, flexible, electrically conductive belt, electric eddy current means positioned adjacent a portion of the conductive belt and operable to cause a magnetic eddy-current field to enter the conductive belt to induce an eddy current in said portion, which eddy current interacts with the field to provide a force to propel the conductive belt toward a surface to-be-imprinted, printing means positioned to move toward and to strike the surface to-be-imprinted under the impetus of the conductive belt, the electric eddy current means comprising a driver plate of non-conducting material containing an embedded electrical conductor extending substantially throughout the whole surface region thereof so that, when the conductor is energized, eddy currents are induced substantially uniformly throughout that portion of the conductive belt adjacent the driver plate, the driver plate being relatively massive compared to the mass of the conductive belt so that the recoil energy of the conductive belt is absorbed mostly by the driver plate, the driver plate and said portion of the conductive belt being closely spaced from one another so that the eddy currents provide an impulse distributed substantially uniformly over the entire said portion of the conductive belt thereby forcing that portion of the conductive belt toward the surface to-be-imprinted.
 11. An eddy-current printer as claimed in claim 10 in which the conductive belt and printing means are associated in a composite structure made of a plurality of thin closed-loop bands, the band adjacent the surface to-be-imprinted being the image carrier of the printer and the other of the bands having high electrical conductivity.
 12. An eddy-current printer as claimed in claim 11 that includes means for applying a transferable image to the image carrier.
 13. An eddy-current printer as claimed in claim 12 in which the image carrier contains indicia and in which the means for applying a transferable image comprises inking means.
 14. An eddy-current printer as claimed in claim 13 that includes a backing plate, the surface to be imprinted being positioned between the driver plate and the backing plate to be pressed therebetween upon the introduction of electric current to the electrical conductor.
 15. An eddy-current printer as claimed in claim 14 that includes a source of pulse current electrically connected to said conducor.
 16. An eddy-current printer as claimed in claim 15 which includes roller means adapted to drive the belt along a closed-loop path, inking occurring at one position of the belt along the path and printing occurring at another position.
 17. An eddy-current printer as claimed in claim 10 said conductive belt being flexible enough to conform to vaRying curvature on the surface to-be-imprinted under the impetus of said impulse. 